Page 136 - Handbook of Battery Materials

104 3 Structural Chemistry of Manganese Dioxide and Related Compounds

compounds can be linked to each other resulted in the occurrence of manganese
oxides with tunnel sizes ranging from [1 × 1] (pyrolusite) to the very large [3 × 3]
channels present intodorokite.The latest investigations of the rubidium manganese
oxides Rb 0.27 MnO 2 and Rb 16.64 Mn 24 O 48 [19–21] demonstrated that even [2 × 4] and
[2 × 5] channel structures do exist. Furthermore, the HRTEM investigations of
Turner and Buseck [4] confirmed the intergrowth model of De Wolff for γ -MnO 2
and it could also be shown that an intergrowth of larger tunnel structures may
occur. The authors reported the occurrence of intergrown todorokite [3 × 3] with
[3 × 7] structures as well as random arrangements of hollandite ([2 × 2], α-MnO 2 )
with roman` echite units ([2 × 3] tunnels). Therefore they proposed a classification
scheme (see Table 3.2) which describes the crystal structures as a system of
tunnels T(m, n) with a common dimension m and a variable dimension n.For
example, pyrolusite, which contains only –[1 × 1] tunnels, is denoted as T(1, 1), and
a compound that contains [2 × 2] (hollandite-like) and [2 × 3] (roman` echite-type)
tunnels can be considered as an inter-growth of T(2, 2) and T(2, 3) types. With
increasing n the structures approach the layered compounds (e.g., T(2, 4) and T(2,
5) structures with broad channels). Finally, structural features denoted as T(1, ∞)
and T(2, ∞) can be regarded as representatives of phyllomanganates.

3.3
Layer Structures
Similarly to the tunnel structures of β-MnO 2 , ramsdellite, γ -MnO 2 , the layered
manganese dioxides, and many related compounds are based on a more or
less distorted hexagonal close packing of oxygen atoms. In layer manganates or
phyllomanganates, the manganese atoms occupy the octahedral voids in such a way
that two-dimensionally infinite sheets of edge-sharing MnO 6 groups are formed. In
the direction perpendicular to the layer plane, the empty and filled layers alternate.
In general, the structure of layered manganese oxide is similar to the C6 type (CdI 2
or Mg(OH) 2 (brucite) structure). A schematic drawing of this lattice type, consisting
of stacked layers formed by edge-sharing octahedra, is shown in Figure 3.8.
The enormous variety of manganese oxides with structures similar to the one
shown in Figure 3.8 arises from the different cation and water contents of the

Figure 3.8 Schematic drawing of the lay-
ered manganese oxides. The structure con-
sists of a stacking of empty and Mn(III, 1V)
-tilled layers of edge-sharing octahedra.